The present invention relates to a control method and a control apparatus for an electric power pump type power steering system in which a brushless motor is used to drive a hydraulic pump.
In general, in an electric power pump type power steering system used in a vehicle, an electric motor drives a hydraulic pump in response to a vehicle running conditions such as a vehicle velocity, and a desired assist force is obtained by the hydraulic pressure of the hydraulic pump.
Conventionally, brush motors have been used in many cases as electric motors to be used in such an electric power pump type power steering system. However, in view of the fact that a friction is generated in brushes in contact with rectifiers and it is necessary to periodically inspect and maintain the brushes, brushless motors that dispense with brushes have recently been used.
FIG. 17 is a view showing a structure of a driver for driving such brushless motors, in which reference numeral 1 denotes a three-phase brushless motor, reference numeral 2 denotes a battery, reference numeral 3 denotes a DC-DC convertor, reference numeral 4 denotes a gate driver circuit, and reference characters Q1 and Q2 denote field effect transistors (FETs) for supplying armature coils L of the brushless motor 1 with electricity.
Incidentally, in FIG. 17, there is shown an example of drive for a single phase out of the three ones of three-phase brushless motor. The systems for the other two phases are driven by the same structures. Also, in the drawings described hereinafter, the description is made in the same manner.
In this case, in the case where the motor 1 is driven, due to the fact that in some drive conditions a current is caused to flow from a terminal +V of the battery through the upstream transistor Q1 to the motor coil and in other drive conditions a current is caused to flow from the motor coil through the downstream transistor Q2 to a terminal GND of the battery, the transistor Q1 is referred to as an upper transistor and the transistor Q2 is referred to as a lower transistor.
Namely, in the case where the brushless motor 1 is driven, first of all, the upper transistor Q1 is turned on, and thereafter the other lower transistor (not shown) is turned on, whereby a current is caused to flow in a direction from armature coils L1 to L2 for the armature coils L1 and L2, for example. Subsequently, after the upper transistor Q1 is turned off, the upper transistor (not shown) is turned on and at the same time the lower transistor Q2 is turned on, whereby a current is caused to flow in a direction from the armature coils L2 to L1 for the armature coils L1 and L2. Thus, the current is caused to flow to thereby rotate the motor 1 by changing, in order, the directions for the armature coils L1 to L3 by the respective transistors Q1 and Q2 and the other transistors (not shown). By the way, in the case where the motor 1 is driven, the upper transistor Q1 and the lower transistor Q2 connected in series with the upper transistor Q1 are controlled not to be turned on simultaneously.
By the way, in the case where an N-channel element like the field effect transistor Q1 is used as the upper drive element of the brushless motor 1, it is necessary to set, at a higher voltage level, the control voltage that is a gate voltage than the drive voltage that is a source voltage. For this reason, in the example shown in FIG. 17, the DC-DC convertor 3 is used to thereby increase the gate voltage of the field effect transistor Q1. Namely, when an instruction for turning the transistor Q1 on is executed by a gate drive signal, the gate driver circuit 4 applies +voltage of a terminal OUT of the DC-DC convertor 3 to a point between the gate and source of the transistor Q1.
Thus, in the conventional brushless motor driver, in the case where the control voltage of the upper transistor is set at a higher voltage than the drive voltage, the DC-DC convertor is used. However, this DC-DC convertor normally operates during the period when the brushless motor driver operates, this is not preferable in an energy efficiency point of view. Also, since the DC-DC convertor has to be provided for every driver circuit of the three-phases of the DC-DC convertor, there are problems that the structure thereof is complicated, and also that the system is expensive.